One of the major challenges in gene therapy for AIDS as well as other diseases is to achieve more efficient transduction of CD34+ hematopoietic progenitor cells and maintenance of efficient expression following differentiation into mature progeny cells. Most current protocols utilizing murine retroviral vectors have proven to be relatively inefficient for transduction of human stem cells and for maintenance of long term and efficient expression of differentiated human T-cells. We propose to model novel approaches to stem cell gene transfer utilizing primate and human retroviral sequences. We will use the human immunodeficiency virus (HIV) and related simian immunodeficiency virus (SIV) genomes to develop vectors that will be more efficient for transduction of human stem cells and expression in mature T-cells. Potential advantages of utilizing human retrovirus vectors are: 1) efficient and sustained expression in the cells that are the target of HIV- 1, mature CD4+ T cells; 2) the ability to establish infection of cells at different points in the cell cycle; and 3) the potentially inducible nature of such vectors. Of particular importance we have developed an in vivo model for stem cell gene therapy utilizing the severe combined immunodeficient (SCID) mouse reconstituted with a human self-renewing thymus (SCID-hu). We have shown that this model allows transduction of CD34+ cells, leading to differentiation into mature T-cells containing the vector. The SCID-hu mice can be infected with HIV-1, resulting in T-cell depletion. Thus, it will be possible for us to test these vectors both in the laboratory and in a self-reconstituting hematopoietic environment in vivo to assess efficacy as well as potential toxic effects upon lymphoid cell differentiation. The Specific Aims are to; 1 Develop constitutively expressing and inducible vectors for stem cell gene transfer based upon the HIV and SIV genomes; 2) In vivo modeling of vector strategies; evaluation of persistence, expression and effects on lymphoid differentiation in the SCID-hu model; and 3) Following introduction of model anti-HIV gene therapeutic reagents such as ribozymes into the above vectors, we will retest vector efficiency and determine efficacy against HIV-1 infection in T-cells in vitro and in SCID-hu mice.